What is a sleep calculator?
A sleep calculator is a tool that works backward or forward from a fixed time to suggest when to fall asleep or when to wake up, counting sleep in roughly 90-minute cycles and adding the minutes you spend drifting off, so your alarm lands between cycles instead of inside one.
How to use it
Two modes, one number each. Pick whichever matches the situation you're actually in at the moment you're reading this.
- 1
Pick your mode
Working backward from something fixed (a flight, a shift, first period)? Use "I want to wake up at." Already in bed at midnight, negotiating with yourself? Use "I'm going to bed now" and see what the alarm can honestly be set to.
- 2
Set how long you take to fall asleep
The default is 15 minutes. If you're out before the podcast finishes, drag it down. If you lie there relitigating a conversation from 2019, drag it up. Nobody else's calculator lets you touch this, and it's the number that changes every result.
- 3
Take the closest time and commit
Pick the row that gives you the sleep you need, not the one that lets you stay up. Then actually set the alarm, and put the phone somewhere you have to stand up to reach.
Is the 90-minute sleep cycle a myth?
It's an average that got promoted to a rule. In "Normal Human Sleep: An Overview" (Principles and Practice of Sleep Medicine), Mary Carskadon and William Dement put a full cycle at 70 to 120 minutes. It shifts from person to person, and it shifts inside a single night: cycles stretch as morning gets closer, so your last one is longer than your first. Ninety minutes is the middle of that range, not a measurement of your brain.
Which is why every calculator on the internet, including this one, gives you a target and not an appointment. Aim within about 15 minutes of a suggested time and you've done everything the science supports. Chasing the exact minute is precision the underlying research doesn't have, and staying up 40 minutes past a "perfect" bedtime to hit the next one is a bad trade every time.
| Age | Recommended sleep | Cycles at 90 min |
|---|
| Children 6–12 | 9 to 12 hours | 6 to 8 |
| Teens 13–18 | 8 to 10 hours | About 5 to 7 |
| Adults 18–60 | 7 hours or more | 5 or more |
| Adults 65+ | 7 to 8 hours | About 5 |
Source: American Academy of Sleep Medicine, 2015
The calculator can't get you out of bed
This tool does arithmetic. It has no opinion about 6:00am, which is where the plan usually dies. A 2023 study in the Journal of Sleep Research found that snoozing costs about six minutes of sleep and left no measurable dent in cognition, so the snooze button isn't wrecking your brain. It's making you late, six minutes at a time, until it's 6:40 and last night's plan is gone.
Risly is the alarm we built for that gap. There is no snooze button anywhere in the app, and the alarm only goes quiet once you've completed a mission: scan an object across the room, solve math, shake the phone, do push-ups. It's iOS 26 and up, built on Apple's AlarmKit, so Android is out for now. And if what you want is to be eased awake, buy a sunrise lamp instead — this app is the opposite of that on purpose.
Frequently asked questions
How many sleep cycles do I need per night? +
Five or six, if you use the 90-minute average — that's 7.5 to 9 hours. The American Academy of Sleep Medicine's 2015 recommendation is the simpler version: adults 18 to 60 should get 7 hours or more. Count the hours first and the cycles second.
Is 6 hours of sleep enough? +
For most adults, no. Six hours is exactly four 90-minute cycles, so it can feel cleaner than 6.5, but the American Academy of Sleep Medicine's 2015 recommendation for adults 18 to 60 is 7 hours or more. Landing on a cycle boundary doesn't buy back the missing hour.
What time should I go to bed to wake up at 6am? +
10:15pm gets you five cycles (7.5 hours) and 8:45pm gets you six (9 hours), assuming about 15 minutes to fall asleep. For most adults 10:15pm is the realistic target: it clears the 7 hours the AASM recommends.
Is the 90-minute sleep cycle real? +
It's real as an average and wrong as a rule. Carskadon and Dement's overview of normal human sleep puts a cycle at 70 to 120 minutes, and cycles get longer toward morning. Use 90 minutes to pick a target, then allow about 15 minutes of slack on either side.
Why am I still tired after 8 hours of sleep? +
Usually sleep inertia — the fog right after waking, which Tassi and Muzet (Sleep Medicine Reviews, 2000) measured at 15 to 60 minutes. Eight hours doesn't skip it; it just wears off. If the tiredness lasts all day, every day, that's a question for a doctor, not a calculator.
